1. Field of the Invention
The present invention generally relates to the reception of RF (radio-frequency) signals, and more particularly, the present invention relates to an RF receiver device equipped with an automatic gain control function that controls an amplification gain (amplification factor) of a RF amplification stage.
Herein, “radio-frequency” is broadly defined and is not particularly limited. Non-limiting examples include high-signal frequencies for broadcasting of high-definition television (HDTV) signals, such as the ultrahigh frequency (UHF) bands utilized in the Integrated Service Digital Broadcasting—Terrestrial (ISDB-T) standard.
2. Description of the Related Art
A conventional ISDB-T type RF receiver is illustrated in FIG. 1, and includes an RF variable gain amplifier 1, a mixer 2, an intermediate-frequency (IF) variable gain amplifier 3, a filter 4, an ADC (analog-digital converter) 5, power detectors 6 and 7, and an IF gain controller 8. Receivers of this type are disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-290178 and Japanese Patent Application Laid-Open No. 2007-019900.
The RF variable gain amplifier 1 includes a gain control terminal for varying an amplification gain of an input RF signal received via an antenna (not shown). The resultant amplified RF signal is supplied to the mixer 2, which is configured to mix the supplied RF signal with an oscillation signal generated from a local oscillator to convert the RF signal into an intermediate-frequency (IF) signal. The IF variable gain amplifier 3 includes a gain control terminal for varying the gain the IF signal received from the mixer 2. The filter 4 extracts (passes) intermediate frequency signal components from the output of the IF variable gain amplifier 3. The ADC 5 is configured to convert an analog IF signal output from the filter 4 into a digital IF signal. The digital IF signal is then supplied, for example, to an OFDM decoder circuit (not shown).
The power detector 6 is configured to detect a power level of the IF signal output from the mixer 2, and supply a corresponding gain control signal to the gain control terminal of the RF variable gain amplifier 1. The RF variable gain amplifier 1 changes the amplification gain thereof in accordance with the gain control signal.
Likewise, the power detector 7 is configured to detect a power level of the digital IF signal output from the ADC 5 and to supply corresponding power level detection signal to the IF gain controller 8. The IF gain controller 8 is configured to generate an gain control signal in accordance with the power level detection signal and to supply the gain control signal to the gain control terminal of the IF variable gain amplifier 3. The IF variable gain amplifier 3 changes the amplification gain thereof in accordance with the gain control signal.
By adjusting the respective amplification gains of the RF variable gain amplifier 1 and the IF variable gain amplifier 3, the IF signal can be controlled to a suitable level and supplied to a decoder circuit when an RF signal of a desired channel is received.
In the conventional RF receiver device, a reception frequency band of the RF variable gain amplifier 1 contains therein a broadcast wave component for each of a plurality of channels, and a broadcast wave component of a desired channel is selected by the mixer 2 from among the plural channels. The broadcast wave component of the desired channel is centered on an intermediate frequency. As illustrated in FIG. 2, the situation can occur in which the reception frequency band of the RF variable gain amplifier 1 further contains therein broadcast wave components of channels adjacent to the broadcast wave component of the desired channel. In such a case, as also illustrated in FIG. 2, in the frequency domain of an output IF signal of the mixer 2, the broadcast wave component of the desired channel, i.e., a broadcast wave component of an intermediate frequency, and broadcast wave components of plural adjacent channels in the vicinity of the desired channel are present. Since the broadcast wave components of the plural adjacent channels are included in a power detection range of the power detector 6, respective power levels of the broadcast wave components within the power detection range are reflected on the power level detection of the power detector 6. Therefore, even when a broadcast wave exhibits a high reception power level, the amplification gain of the RF variable gain amplifier 1 is controlled.
On the other hand, as illustrated in FIG. 3, the situation can occur where the reception frequency band of the RF variable gain amplifier 1 contains therein broadcast wave components of channels distant from, and not adjacent to, the broadcast wave component of the desired channel. In such a case, as also illustrated in FIG. 3, in the frequency domain of the output IF signal of the mixer 2, broadcast wave components of plural channels outside the power detection range of the power detector 6 and distant from the broadcast wave component of the desired channel are present. Since only the broadcast wave component of the desired channel is included in the power detection range of the power detector 6, the power level of only the broadcast wave component of the desired channel is reflected on the power level detection of the power detector 6. Therefore, the amplification gain of the RF variable gain amplifier 1 is controlled in accordance with the power level detection of only the broadcast wave component of the desired channel.
When the reception signal power of the broadcast wave component of a channel distant from the desired channel is large, the RF variable gain amplifier 1 might be saturated with the reception signal power of the distant channel. As a result, the signal component of the desired channel may be distorted, and thus, reception performance may deteriorate.
To overcome this problem, the gain of the RF variable gain amplifier 1 may be set to a low value. However, in this case, when other channels adjacent to the desired channel are present, power levels of other channels other than the desired channel are reflected on the power detection. Therefore, the power of the signal component of the desired channel after having passed through the filter 4 becomes very low, and thus, there may be a case where it is difficult to obtain sufficient signal amplitude for decoding.
On the other hand, in a case where the power detector is provided at an input of the mixer 2 instead of the output of the mixer 2, the gain of the RF variable gain amplifier 1 is controlled by the entire reception power present in the reception frequency band. Therefore, as the sum of the power of other channels present in a region where saturation rarely occurs increases, an amplification factor of the desired channel decreases. As a result, the case can arise in which is difficult to obtain a signal amplitude required for decoding. In an effort to minimize this problem, the gain of the RF variable gain amplifier 1 may be set to a high value. However, in this case, it is not practically possible to detect saturation occurring in the mixer 2, and thus, the RF variable gain amplifier may enter into an unintended saturation state.